The last three posts have described what happens when a jet of water first arrives on a surface and then starts to penetrate into the material. At a close stand-off distance, the erosion starts around the edge of the jet and continues to widen the hole as it gets deeper, until a point where the pressure at the bottom of the hole falls and the jet stops going deeper. The lateral flow away from the bottom of the jet continues to erode material, however, and so the hole gets a little wider at the bottom. This creates a small chamber under the entrance hole and this can build up enough pressure that it can cause the material around the hole to break.

Figure 1. Progress in the high-pressure waterjet drilling of a hole in rock.

In the last post I showed where this happened with a 1-ft cube of rock that had been broken with a single pulse, but this fracture of the target can occur when piercing glass or other brittle materials. So the question becomes how to stop the fracture if one is trying to cut glass. This applies when the job calls for making an internal cut in the glass, and not when cutting in from the side, although that also has some problems that I will address in a later post.

When starting an internal cut, it obviously means piercing a starter hole through the glass in a region that is going to be part of the scrap, if this is possible, as it would be, for example, when cutting a sculpture. A secondary reason for that location, apart from confining any small cracks that might happen during the pierce, is that these starter holes are larger in diameter (for the reason given above) than the cut line once the jet starts to move, and that hole section would appear as a flaw on a final cut line.

Vanessa Cutler, in New Technologies in Glass discusses the process of cutting in more detail but suggests that the starter hole be pierced at a lower pressure than that to be used in the cut. This is so that the pressure within the cavity will remain lower during the pierce, and insufficient to cause the glass to break. She suggests (and she has a vastly greater experience than I in this) that the piercing pressure be around 11,000 to 18,000 psi – this varies a bit with abrasive grit size, machine size and glass type.

Figure 2. Detail of the glass sculpture “p1″, by Vanessa Cutler. (Note that these holes do not pierce all the way through the glass but all end at the same depth.)

She also recommends, when there are multiple cuts to be made on a sheet, that all the piercing holes be completed before any cutting begins. One of the reasons for this is to avoid constantly resetting the cutting pressure, which could be a problem if you forget to lower the pressure back down before starting the next pierce. (Would I as an Emeritus Professor ever be that absent-minded? Why else bring it up?)

You will notice, with abrasive cutting into glass, that there is not the belling at the bottom of the cut like with plain waterjet cutting and that the hole tapers with depth as the cutting effectiveness reduces with the fall in pressure with depth; and the jet is less able to cut into the side walls of the opening at these lower pressures.

Stepping back from the cutting of glass to the more general condition where the jet runs out of power at the bottom of the hole, the main reason for this is the conflict between the water in the fresh jet coming into the hole and the spent water trying to make it out of the hole at the same time.

One way of overcoming the problem is to interrupt the flow of water into the hole. Back in my grad student days, we tried doing this by breaking the jet into slugs, so that one slug would have enough time to travel to the bottom of the hole, cut a little, and then rebound out of the hole, before the next slug of water arrived. There was relatively little sophistication in the tool we designed to do this. Simply it was a disk, with holes drilled in it at an angle.

The reason for the angled holes was to make the disk self-propelling as it rotated under the jet, since the angled edges of the hole forced the disk to continue rotating once started. (On a minor note, the disk would rotate at several thousand rpm, and the noise that it made was loud enough that I was instructed to only carry out the tests after the staff had left for the evening).

Figure 4. The penetration of a waterjet into sandstone with the jet running continuously (black), with the jet interrupted (red) and with the jet rotated slightly off-axis (green). (Brook, N. and Summers, D.A., “The Penetration of Rock by High Speed Waterjets”, Intl. Journal Rock Mechanics and Mining Science, May, 1969)

As can be seen in figure 4, with the pulsating jet more energy was getting to the bottom of the hole without interference, and the hole continued to deepen over time. However, the interruption tool had a number of disadvantages, apart from the noise and that the disk would be very rapidly destroyed under an abrasive jet. It was wasting a significant portion of the energy: In a more optimized design that I won’t discuss further, the energy loss was about 50%.

So it would be best if the jet was moved slightly over the surface, and in these early tests, the easy way to do this was to have the target rotate with the jet hitting the rock just offset from the axis of rotation. (At the time high-pressure swivels weren’t yet available). This gave the upper curve in figure 4, and a much more rapid penetration of the target.

In more modern times, the nozzle is moved either by causing it to move slightly around the hole axis or by causing a slight oscillation or “dither” in the nozzle while the pierce is taking place. This is generally a feature of the control software that drives the cutting table. But the reason for the movement is to get the water flowing in such a way that the water going out of the hole does not interfere with that going in, and so there is a reduced risk of pressure build-up in the hole, with the consequent cracking that this would cause.

Here’s a little demonstration you can carry out. Take a strip of paper, and cut a slit in it half way along the strip and half way through the paper. Now take both ends of the paper in your hands and pull them apart. This causes the cut (the crack) to grow through the paper and gives you two halves. If you do this a second time you should find that by stopping moving your hands you can stop the cut from growing all the way through the paper. Now repeat the process, but use a piece of paper that you have not cut a slot in. The amount of force you need to pull the paper apart is much higher, and I seriously doubt that once you get the tear (crack) to start that you can stop it before it goes all the way through the paper. (Remember this, and I’ll come back to it a bit later in time).

The idea of putting cracks on the edge of packages to lower the force you need to tear them open can be found on the edge of lots of candy bars, packs of peanuts and other goodies in stores. The serrated edge acts as a series of cuts or cracks, that concentrate the force applied when you pull on the edges of the packet so that the package tears at a much lower force, and you can control the tear so that you don’t end up throwing all the contents around the room.

Figure 1. Serrations and tear at the top of a packet of honey

Now at this point you might say that there aren’t any cracks in glass when we start to cut it. If the glass is very new, this is true. However, with all the chemicals in the air and the dust that is carried in the wind the surface actually contains a lot of very fine cracks although glass can look clear.

John Field, one of the earlier investigators of high-pressure waterjet impact, showed this in one of those brilliant yet simple demonstrations that, in this case, he carried out some forty-five-odd years ago. If waterjet impact grows surface cracks and glass acquires surface cracks from damage through being out in the air and if that surface layer is removed, then the underlying glass will have no cracks. So John took a glass slide, and etched off the surface of the lower half of the slide, by immersing it in acid. Then he fired a very high-speed droplet of water at the point on the slide where the acid etch stopped.

Figure 1. Impact of a high-speed droplet of water on glass. Above the dividing line the glass surface contains the micro-cracks and flaws that come with being exposed to the air over time. The lower section below the line has had these flaws removed. As can be seen the cracks only develop in the unetched part of the glass, where they grow pre-existing cracks, even into the side of the glass that was etched. (Field J.E. “Stress Waves, Deformation and Fracture Caused by Liquid Impact,” Phil. Trans. Royal Society, 260A, July 1966, pp. 86 – 93.)

In a single picture he captured the evidence that waterjets work by growing cracks (top half), and that without cracks there is no damage (bottom half). Understanding this opens up a whole vista of different applications, from the removal of soil from around pipelines underground (the new technology of hydro-excavation) to the removal of damaged concrete, while leaving healthy concrete in place (the developed field of hydro-demolition). These and other topics will be part of this series as it moves forward.

But as John showed, not all the cracks a jet will grow can be seen, and as Vanessa found, they don’t have to be at the surface to create problems. One of her early pieces was entitled “p1.” Within it are an uncountable series of holes, drilled deep into the glass.

Figure 2. Detail of the glass sculpture “p1”, by Vanessa Cutler

One of the skills Vanessa has learned is in controlling the quality of the pierce and its dimension, but initially, there had to be a period of learning.

Figure 3. Single cracks growing out from partial piercings in a test piece during development (Vanessa Cutler)

And so, in the next sequence of posts the simple idea of growing existing cracks will be explored. Mainly, in the beginning, this will focus on cracks that are already there, and how to usefully make them grow. But in some cases we don’t want all those cracks to grow, and that will also come up, as this series continues.

High pressure abrasive waterjets (AWJ) are able to cut glass with considerable precision, and maintain the accuracy of cut through thick material.

Figure 1. Cutting an Eagle from glass using AWJ (courtesy of KMT)

Because of this precision, and because the glass can be cut to leave very delicate webs between adjacent cuts, AWJ glass-cutting has been used to create art objects for a number of years. It is not as easy as it might at first seem, and Dr. Vanessa Cutler, an international leader in advanced cutting, has used the tool to create significant works of art. She has also written on the problems that can arise in cutting what often seems to be a simple, consistent material. (Noting, in passing that through the combination of computer control and memory it is easier at times to re-create works that break than would be the case with other tools for artistic creation). For the more mundane cutting world that comprises the rest of us, cutting glass is more likely restricted to simple activities such as cutting the parts for the windows of wood furnaces.

When the cuts are this simple, time can be saved by stacking two or more sheets of glass, one on top of the other, and cutting all of them at the same time. As one learns the parameters, thicker and greater numbers of plates can be stacked, and still successfully cut.

Figure 2. Cutting through four sheets of glass simultaneously (courtesy of KMT)

However, if one gets too ambitious, and stacks too many plates then the lower plates may start to crack, often after the cut has started into the plate. As Dr. Cutler has noted in her new book “New Technologies in Glass”, cracks can also create problems for the unwary in dealing with internal stresses in the structure of the glass.

There can be several reasons for this, but it primarily goes back to the point I made in the introduction, about water pressure causing existing cracks and weakness planes to grow, as a way of removing material. There are two sorts of cracks that exist in glass, those created by the impact of the abrasive particles themselves, and those that were already present in the glass.

Figure 3. Micro-photograph showing cracks growing out from the point where two abrasive particles struck a piece of glass. (This was adjacent to the main cutting path).

Figure 4. Micro-photograph of the edge of the main cut by an AWJ on glass, showing that it is made up of the intersection of adjacent cracks created by the abrasive impact.

I’ll write about the mechanics of cutting glass in a later post or two, but for the moment I would like to write about the basics of crack growth from the point of view of cracks that already exist in the material. In large part this won’t be using waterjets alone to cut glass. Rather there are lots of other materials, particularly soil and rock, which have much higher crack densities, and longer cracks which make it easier to cut and remove material.

So, for the next four KMT Waterjet Blogs, I will focus the issues of cracks in materials including cutting glass, cutting granite and much more.